This invention relates to novel solder compositions or solder creams, and is particularly concerned with the provision of solder compositions especially adapted for soldering components in high-density electronic circuitry.
A solder cream is a material that is capable of being applied to a substrate or surface in a specific pattern using screening or analogous methods which can subsequently undergo fusing to provide an electrical joint or interface commonly described as a solder joint. The solder cream generally consists of metallic balls of various alloys such as tin-lead, tin-lead-silver, tin-lead-gold, etc., contained in a vehicle including a flux such as wood rosin, or derivatives thereof. In addition, a suspension medium is present in the vehicle and is combined with the flux to provide a paste substance of uniform texture, consistency, and improved surface wetting. In addition, an organic solvent is present in the vehicle to provide the proper consistency for the vehicle.
In electronic circuitry, solder creams or compositions are employed to secure miniature electronic components to a circuit on a substrate such as ceramic or glass, or the like. Circuits similar to those used on printed circuit boards are produced on the substrates. In certain instances, the circuit is located on one side of the substrate, with pads being provided in certain areas, other parts of the circuit being covered with an insulating material.
The solder cream having a paste-like consistency is applied, for example, by means of a silk screen, to the pads on the circuit board. Thereafter, the electronic components are carefully positioned with their peripheral contacts on the solder cream-coated pads. When all of the components are thus in place, the board, with such components temporarily positioned and retained thereon by the solder cream, can be placed in a vapor reflow system and subjected to a sufficiently high temperature to cause the metal content of the solder cream to liquefy and the contacts of the electronic components to be fused and to adhere to the pads on the circuit board.
My U.S. Pat. No. 4,373,974 discloses a novel solder composition for soldering components in electronic circuitry, whereby the formation of solder balls is essentially eliminated, thus avoiding the necessity of removing such solder balls. Such composition comprises solder metals such as tin and lead, dispersed in a vehicle containing viscosity-controlling agents, organic solvents and a flux, employing a narrow range of between 13 and 14% vehicle, and between 86 and 87% of solder metal, by weight of the solder composition.
Although the composition of my above patent has proved highly satisfactory for soldering components generally in electronic circuitry, electronic circuits of increasingly higher density have been developed recently, and the higher the density of such circuits, the smaller the pad sizes become to which the electronic components are to be connected, and the smaller the electrical connections become. It has been found that in order to achieve efficient fusing of electrical components to the very small pad sizes of the order of 10 mils square or smaller, the composition of the solder formulation must be modified from that disclosed in my above patent. In the solder composition of my above patent, the particle size of the solder metal ranges from 40 to 70 microns, corresponding to a range of approximately 1.5 to about 3 mils. Therefore, the particle size of the solder metals of my above patent is too large, creating difficulties in fusing electronic components to small pads of the sizes noted above. On the other hand, as the particle size of the solder metals is reduced, the ability of the vehicle including the flux to hold the powder in suspension in the composition presents a problem.
It is accordingly an object of the present invention to provide a solder composition or solder cream which is especially adapted for soldering components in high-density electronic circuits.
Another object is the provision of a solder composition of the above type which minimizes formation of undesirable solder balls causing short circuits between electronic components.
A still further object of the invention is to provide an efficient solder composition of the type disclosed in my above patent, and which is particularly adapted for soldering electronic components to small pads in high-density electronic circuits.